Reversal of Input Tax Credit: interim certificate requirement for post supply discounts to validate exclusion from taxable value. Where suppliers issue tax credit notes post supply, the discount is excludable from taxable value only if it meets the Section 15(3)(b) conditions including proportionate reversal of input tax credit by the recipient; until portal verification exists, suppliers must obtain a CA/CMA certificate (with UDIN) from the recipient detailing credit notes, relevant invoices, ITC reversal amounts and supporting FORM GST DRC 03/return evidence, or where tax involved is below a low threshold, an undertaking from the recipient; such documents constitute admissible evidence and must be produced to tax officers on demand.